Time of flight (TOF) mass spectrometers are widely used to determine the mass to charge ratio (m/z) of ions on the basis of their flight time along a flight path. Ions are emitted from a pulsed ion source in the form of a short ion pulse and are directed along a prescribed flight path through an evacuated space to impinge upon or pass through an ion detector. The detector then provides an output to a data acquisition system. The ion source is arranged so that the ions leave the source with a constant kinetic energy and reach the detector after a time which depends upon their mass, more massive ions being slower. The ion pulse emitted from the source is thus separated along the flight path so that the ions arrive at the detector in a plurality of short ion packets, each packet comprising one or more ions of a particular mass (m/z) or restricted mass range and being typically a few nanoseconds (ns) long. The detector is therefore required to resolve ion packets on this timescale. The detector is typically of a secondary electron emission type so that the ion packets produce electron packets at the detector which get amplified by secondary electron emission by a factor typically of 105-108. If the number of ions in the packets varies over a large range from one packet to another, then saturation of the detector and/or the data acquisition system can take place. If the gain of the detector is reduced to avoid saturation by the most intense ion packets then the detector may not be sensitive enough to detect the least intense ion packets. Thus, the dynamic range of the detector becomes compromised. Moreover, the detector life may be reduced by the effect of intense ion packets.
Currently, the following techniques are known for extending dynamic range of detection in TOF mass spectrometry.
In EP1215711, a method is described which involves switching the transmission of ions prior to extraction in subsequent scans. This method, however, reduces sensitivity and does not protect the detector from intense ion packets.
Another approach is on-the-fly modulation of ion packets following intermediate detection of the ion packets, as described for example in U.S. Pat. No. 6,674,068; and WO 2008/046594. This approach has the drawbacks that it requires an additional detector and more than one temporal focal point in the flight path, which is not feasible for some types of flight paths.
Splitting of the ions onto two or more detectors is described in U.S. Pat. No. 7,126,114 and US 2002/0175292. Such arrangements where the detectors have different gains and the detector outputs can be combined are described in U.S. Pat. No. 6,864,479 and U.S. Pat. No. 6,940,066. In addition to requiring two or more separate detectors, there is also no protection of the detector from intense ion packets in these arrangements.
Still further methodologies are known, including splitting of the electron packets produced by the ions between multiple anodes of similar dimensions (as described in U.S. Pat. No. 5,777,326) or different dimensions (as described in U.S. Pat. No. 4,691,160; U.S. Pat. No. 6,229,142; WO99/38191; U.S. Pat. No. 6,646,252); expansion of electron packets over a greater number of amplification channels (as described in U.S. Pat. No. 6,906,318 and U.S. Pat. No. 7,141,785); and detection of electron packets using two or more data acquisition channels with different gain.
Almost all of these techniques offer no protection of the detector from intense ion packets, an exception being the on-the-fly modulation of ion packets. However, an increase of ion transmission from the ion source through TOF analysers from the current few percent in today's systems to potentially greater than fifty percent in future systems will mean that the ion flux onto the detector could go up to >108 ions/second. This would reduce lifetime of detector to unacceptable levels (e.g. a few hours) and therefore needs to be addressed.
On-the-fly modulation of detector gain is described in WO2006/014286 (U.S. Pat. No. 7,238,936) in relation to slower scanning mass spectrometers than TOF mass spectrometers where there is sufficient time for an intermediate stage of detection to disable a subsequent stage of detection and the speed of modulation is on the scale of milliseconds or microseconds. In such a prior art device, the rise time of an incoming ion signal (e.g. during a mass scan in a quadrupole, RF-ion trap or sector MS) is sufficiently long that a dynamic switching that acts on later arriving ions is sufficient to adequately modulate the signal. The detectors described therein would however not be suitable for detecting ions in a TOF mass spectrometer or faster scanning mass spectrometer where rise and fall times of the signals due to the incoming ion packets are typically of the order of a few nanoseconds (ns) long.
Accordingly, there remains a need to improve the detection of charged particles in TOF mass spectrometry. In view of the above background, the present invention has been made.